Traveling speed of a vehicle can be classified into high-speed over 100 km/h, medium-speed from 50 km/h to 100 km/h and low-speed below 50 km/h. Table 1 shows frequency characteristics of vehicular noise according to the traveling speed.
TABLE 1TRAVELLINGNOISEMAJOR NOISESPEEDFREQUENCYSOURCELOW-SPEED30 Hz-60 HzENGINE TORQUEMEDIUM-SPEED 60 Hz-100 HzENGINE TORQUEHIGH-SPEED100 Hz-120 HzENGINE TORQUE
As shown in Table 1, vehicular noise occurring during vehicle driving is mainly caused by an engine torque, and has different frequency characteristics according to the traveling speed. The vehicular noise due to the engine torque is characterized in that its energy is concentrated within a relatively low frequency band.
Besides the engine torque, interference is increased with an increase of the traveling speed due to various noise sources, e.g., friction between the road surface and tires, impact due to an irregular road surface and friction between a vehicle and the air. The vehicular noise due to the above-described various noise sources has time-varying frequency characteristics and, unlike the vehicular noise due to the engine torque, is characterized in that its energy is widely distributed over frequency bands, thereby serving as a major factor of distortion of an input speech signal and deterioration of performance of a speech recognition system. In general, speech recognition performance is significantly deteriorated when the traveling speed is over 100 km/h.
As for the frequency characteristics of the vehicular noise, a frequency band of impact sound due to a vehicle engine cycle is in a range from 40 Hz to 200 Hz, a frequency band of friction sound between the road surface and tires is in a range from 100 Hz to 1000 Hz, a frequency band of resonant noise of engine sound is in a range from 300 Hz to 500 Hz and a frequency band of impact sound due to the irregular road surface is in a range from 30 Hz to 60 Hz. Meanwhile, frequency characteristics of noise due to external air flow during vehicle driving is similar to that of white noise, i.e., such noise is not limited to a specific frequency band but distributed over the entire frequency band.
Additive noise for a vehicle traveling at the low-speed or medium-speed relatively less distorts a speech signal, because it is concentrated within relatively narrow low frequency bands. However, additive noise at the high-speed becomes a major factor of distortion of the speech signal, because it is dispersed over relatively wide frequency bands.
Conventional speech recognition systems embedded in a vehicle have problems caused by an increase of driving noise, e.g., distortion of an input speech signal, performance degradation of a VAD (Voice Activity Detector) and performance degradation in end point detection. Further, the conventional speech recognition systems employ speech recognition decoders based on a single acoustic model without taking the traveling speed of the vehicle into consideration. Therefore, the conventional speech recognition systems cannot show excellent recognition result.